Electrophotography is a useful process for printing images on a receiver (or “imaging substrate”), such as a piece or sheet of paper or another planar medium (e.g., glass, fabric, metal, or other objects) as will be described below. In this process, an electrostatic latent image is formed on a photoreceptor by uniformly charging the photoreceptor and then discharging selected areas of the uniform charge to yield an electrostatic charge pattern corresponding to the desired image (i.e., a “latent image”).
After the latent image is formed, charged toner particles are brought into the vicinity of the photoreceptor and are attracted to the latent image to develop the latent image into a toner image. Note that the toner image may not be visible to the naked eye depending on the composition of the toner particles (e.g., clear toner).
After the latent image is developed into a toner image on the photoreceptor, a suitable receiver is brought into juxtaposition with the toner image. A suitable electric field is applied to transfer the toner particles of the toner image to the receiver to form the desired print image on the receiver. The imaging process is typically repeated many times with reusable photoreceptors.
The receiver is then removed from its operative association with the photoreceptor and subjected to heat or pressure to permanently fix (i.e., “fuse”) the print image to the receiver. Plural print images (e.g., separation images of different colors) can be overlaid on the receiver before fusing to form a multi-color print image on the receiver.
Electrophotographic printing systems are susceptible to various printing artifacts where the tone reproduction exhibits certain spatial-dependent or temporal-dependent characteristics. For instance, “streak” artifacts can be formed when the tone reproduction of a printing system exhibits spatial variation across the image width. Likewise, “banding” artifacts can be formed when the tone reproduction exhibits temporal variations. Methods to compensate for such artifacts are known in the art and typically involve characterizing the form of the artifacts and introducing compensating modifications to the input images, or the exposure used to print the input images.
Another type of artifact can be formed when tone reproduction variations are caused by the interaction between neighboring imaging pixels. For example, in the electrophotographic printing process, the electric field produced in the uniformly charged area is different from that formed from an area having the same charge in the neighborhood of an image edge. This phenomenon is sometimes referred to as a “fringe field effect,” and the resulting artifacts are sometimes called “halo artifacts” or “fringe artifacts.” Halo artifacts are typically most noticeable in the mid-tone region where the fringe fields generally cause less colorant to be deposited on the receiver media on the lighter side of an image edge than would be deposited in uniform image areas. Similarly, the fringe fields can cause more colorant to be deposited on the receiver media on the darker side of an image edge than would be deposited in uniform image areas. The magnitude of the halo artifacts will vary as a function of the size of the image edge, as well as the coverage levels on both sides of the edge. Generally, the tone reproduction will return to normal behavior for a full strength edge which transitions between an area of 0 percent colorant coverage to an area of 100 percent colorant coverage. The halo artifacts can be characterized as image-dependent tone reproduction variations, where the tone reproduction curve effectively varies as a function of image position.
U.S. Pat. No. 6,606,470 to Wibbels et al., entitled “Color plane partial exposure for reducing edge effects,” discloses a method for reducing edge artifacts in an electrophotographic imaging system. The method includes partially exposing an image region that does not include a color, while normally exposing an adjacent region that includes a color. This reduces lateral electric field effects and the associated edge artifacts.
U.S. Pat. No. 7,016,073 to Meyer et al., entitled “Digital halftone with auxiliary pixels,” discloses a method for reducing halo artifacts that can occur around halftone dots. The method includes adding small auxiliary pixels around the boundary of the halftone dots to modify the fringe field effect. The size of the auxiliary pixels is chosen to be small enough so that they embody frequencies that are beyond the MTF of the printing system, and therefore will not result in toner deposition.
U.S. Pat. No. 7,974,544 to Kobayashi et al., entitled “Electrophotography apparatus having edge detection of toner patch and exposure control,” discloses a method to reduce edge artifacts in an electrophotographic printer. The method compares an image region to predefined templates to identify edge regions. A corrected amount of toner is provided in the identified edge regions based on measurements made for a set of test patches.
U.S. Patent Application Publication 2009/0214238 to Tanaka et al., entitled “Image forming apparatus, image forming method, and image forming program product,” discloses a method for adjusting edge characteristics in a printed image. The method includes comparing a neighborhood of image pixels to a set of reference patterns. The image pixels are modified to produce various effects such as line thinning and line thickening.
U.S. Patent Application Publication 2011/0235063 to Kondo, entitled “Image processing apparatus,” discloses a method for reducing toner consumption in an electrophotographic printer. The method includes analyzing the image to determine an index value relating to an edge intensity around a target pixel. For high edge intensity pixels, the print density is increased, and for low edge intensity pixels, the print density is decreased.
Accordingly, there remains a need for an improved method to efficiently correct for halo artifacts in an electrophotographic printing system.